1. Field of the Invention
The present invention relates to an optical information recording medium, and more specifically to an optical information recording medium writable only once by heat mode.
2. Description of the Related Art
A CD-R is a writable, optical information recording medium (optical disc) on which information can be written only once by irradiation with laser light, and is widely known. A CD-R typically comprises a transparent disc substrate having successively disposed thereon a recording layer including an organic dye, a light-reflective layer including a metal such as gold, and a protective layer (cover layer) made of a resin. Information is recorded on this CD-R by irradiating the CD-R with near-infrared laser light (usually laser light having a wavelength of around 780 nm). Specifically, the portion of the recording layer that is irradiated absorbs light, whereby the temperature rises at the irradiated portion. The rise in temperature produces a physical or chemical change (e.g., formation of pits) to alter the optical properties of the irradiated portion, whereby information is recorded. The information thus recorded on the CD-R is ordinarily reproduced by irradiating the CD-R with laser light having the same wavelength as that of the laser light used to record the information and detecting a difference in reflectance between the region of the recording layer whose optical properties have been changed (recorded portion) and the region of the recording layer whose optical properties have not been changed (unrecorded portion).
In recent years, there has been a demand for optical information recording media having higher density, and writable digital versatile discs (DVD-Rs) have been proposed in response to that demand (Nikkei New Media, extra issue entitled xe2x80x9cDVDxe2x80x9d, 1995). A DVD-R typically comprises two transparent disc substrates that each have successively disposed thereon a recording layer containing an organic dye, a light-reflective layer, and a protective layer, with the discs being adhered so that the recording layers face inward or so that protective substrates having the same disc shape as these discs are disposed on outer sides of the adhered discs. Moreover, the transparent disc substrate includes a guide groove (pre-groove) used for tracking a laser irradiated onto the CD-R, with the groove having a narrow track pitch (0.74 to 0.8 xcexcm) that is equal to or less than half of that in a CD-R. Information is recorded and reproduced (played back) by irradiating the DVD-R with laser light in a visible region (usually laser light having a wavelength region ranging from 630 to 680 nm), whereby information can be recorded at a higher density than a CD-R.
Recently, high-vision television and networks such as the Internet have rapidly become more widespread. In addition, the start of HDVT (High Definition Television) broadcasting is near at hand. As a result, large-capacity optical recording media capable of recording visual information easily and inexpensively are in demand. While DVD-Rs currently play a significant role as large-capacity recording media, the demand for media having greater recording capacity and higher density continues to escalate, and development of recording media that can cope with this demand is also needed. For this reason, development of recording media having ever greater storage capacity with which high-density recording can be effected with short wave light continues to advance.
Methods for recording information on and reproducing information from an optical information recording medium including a recording layer containing an organic dye, by irradiating, from the side of the medium disposed with the recording layer towards the side of the medium disposed with a light-reflecting layer, the medium with laser light having a wavelength of 530 nm or less, are disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028. In these methods, information is recorded on and reproduced from an optical disc having a recording layer containing a porphyrin compound, an azo-based dye, a metal azo-based dye, a quinophthalone-based dye, a trimethine cyanine dye, a dicyanobiphenyl-skeleton dye, a coumarin dye, a naphthalocyanine compound or the like, by irradiating the optical disc with a blue laser (having a wavelength of 430 nm or 488 nm) or a blue-green laser (having a wavelength of 515).
Further, in view of compatibility with CD-R systems currently employed, optical information recording media have been proposed in which information can be recorded and reproduced by two laser beams having mutually different wavelengths. For example, in JP-A Nos. 2000-141900, 2000-158816, 2000-185471, 2000-289342 and 2000-309165, there are proposed optical information recording media with which information can be recorded and reproduced using both a laser beam having a wavelength at about 780 nm in a near-infrared region and a laser beam having a wavelength at about 650 nm in a visible region, by combined use of a dye used in CD-Rs and a dye used in DVD-Rs.
However, the present inventors have found that practically employable sensitivity cannot be obtained with the discs disclosed in the above publications when information is recorded on the discs by irradiating the discs with short-wave laser beam having a wavelength of 600 nm or less, and particularly a wavelength of 450 nm or less, and that sufficient levels cannot been achieved with respect to other recording characteristics such as reflectance and modulation. In particular, it was found that recording characteristics of the optical discs disclosed in the above publications decreased when the discs were irradiated with laser light having a wavelength of 450 nm or less.
In view of the foregoing, it is an object of the present invention to provide an optical information recording medium on which information can be recorded by irradiating the medium with a short-wave laser beam having a wavelength of 600 nm or less, and that exhibits stable playback characteristics.
The object is achieved by the invention described below.
A first aspect of the invention provides an optical information recording medium comprising a substrate including a groove that has a track pitch of 200 to 400 nm and a depth of 20 to 150 nm, the substrate having successively disposed thereon a light-reflective layer, a recording layer containing a dye and on which information is recordable by a laser beam having a wavelength of 600 nm or less, and a cover layer, wherein the recording layer contains at least one organic solvent-soluble compound having an absorption maximum in the range of 300 to 450 nm and an absorbance of 0.07 or more at a recording laser wavelength, and at least one organic solvent-soluble compound having an absorption maximum in the range of 300 to 450 nm and an absorbance of lower than 0.07 at a recording laser wavelength.
A second aspect of the invention provides an optical information recording medium comprising a substrate including a groove that has a track pitch of 200 to 400 nm and a depth of 20 to 150 nm, the substrate having successively disposed thereon a light-reflective layer, a recording layer containing a dye and on which information is recordable by a laser beam having a wavelength of 600 nm or less, and a cover layer, wherein the recording layer contains at least two organic solvent-soluble compounds, each having an absorption maximum in the range of 300 to 450 nm and an absorbance of 0.07 or more at a recording laser wavelength.